Reduction of atmospheric interference



April l, 1930. A. KELLEY RDUCTI-ON OF ATMOSPHERIC` I-NTERFERENCE Filed Nov. 26, 192e INVENTO L'ley A TORNPEY sible,

Patented Apr. 1, l930` UNITED STATES PATENT OFFICE .ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAIH COMPANY, A CORPORATION OF NEW YORK REDUCTION OF ATMOSPHERIC INTERFEREN CE Application filed November 26, 1928. Serial No. 321,996.

This invention relates to radio or Wire tele graph systems, either radio or on wires and more particularly to arrangements for suppressing atmospheric disturbances in such systems.

In accordance with the present invention, it is proposed to utilize, in connection with a double demodulation receiving arrangement, a so-ealled buck channel which tends to prevent false signals upon the part of the receiving relay due to components of the atmospheric disturbance which finally reach the receiving channel.

The double demodulation feature reduces the effect of atmospheric disturbances bylimiting the frequency components which are effective upon the final detector to the frequency range assigned to the receiving channel or channels. Its operation is to first select, so far as is possible at the high receiving frequency, the range of frequencies desired. Sharp selection is not possible at this stage, and hence atmospheric or other interfering components lying somewhat outside the desired range Will also be received by the selecting circuit. The selected range is then stepped down by demodulation to a lower position in the frequency spectrum, and the desired band width for the receiving channel or channels is again Selected by means of a filter or tuned circuit. Due to the lower position of the desired band in the frequency spectrum after being stepped down, it is posat this stage, to more sharply select the wanted frequencies from those which are undesired so that, so far as atmospheric disturbances are concerned, the only components which will affect the final detector are those which lie substantially within the desired band.

As to these components (which cannot be eliminated by selection), the buck circuit goes into play to neutralize or prevent their effects upon the receiving relay. The buck circuit comprises an auxiliary channel associated with the receiving channel beyond the demodulator which steps down the frequency to its intermediate range, such channel being selective of a frequency different from that of the receiving channel or channels but closely related thereto. No carrier is supplied at this frequency from the distant trans`- mitting station, but when an atmospheric dis turhance occurs which has frequency components corresponding to the auxiliary channel, such components are selected and rectified in the auxiliary channel, to produce a current impulse which is applied to the auX- iliary winding of the receiving relay, in such a direction as to tend to hold the armature against the contact upon which it happens to rest at the moment the disturbance occurs. In order that the current impulse be applied in the proper direction, a reversing relay is provided which shifts the direction of the impulse as the received signal changes from marking to spacing, and vice versa.

In general, the double demodulation arrangement and the buck circuit arrangement supplement each other so far as discriminating against atmospheric disturbanees is concerned. The effect of the double demodulation arrangement may be briefly stated to be one of discarding from the band of disturbing frequencies all those lyin without the wanted signal range. The buck circuit then' goes into play to neutralize the effect of those -components of the disturbance which lie Within the Wanted range.

The invention may now be more fully understood from the following description when; read in connection with the accompanying drawing the figure of which illustrates schematically a circuit arrangement embodying the invention.

The apparatus for practicing the invention -comprises a selecting circuit RF S Whose input may be connected to any high frequency receiving circuit such as a high frequency transmission line or radio antenna. For the vpurpose of illustration the selecting circuit 1s shown associated with an antenna. In

the case of a radio system this selecting cir.

cuit may be al tuned circuit. The output side of the selecting circuit is connected to a radio frequency amplifier RFA, which may be of the Well known vacuum tube type, for amplifying the selected band of frequencies. The output of the amplifier is connected to a demodulator DD Whose input circuit is also associated with a heterodyne source G. The demodulator DD may be of any known type of vacuum tube demodulator and the heterodyne source G may, for example, be a vacuum tube oscillator. An intermediate frequency amplifier IFA is connected to the output side of the demodulator DD, this amplifier being of any known type such, for eX- ample, as a vacuum tube.

If the system is a multiplex system, the output of the intermediate frequency amplifier IFA is connected to various selecting circuits, such as IFS, of the several channels, the apparatus of only one channel being illustrated. The selecting circuit IFS may be a tuned circuit, band filter or other known type of selecting arrangement arranged to select from the output of the amplifier IFA the desired band of frequencies for the particular receiving channel illustrated.

The receiving channel includes, in addition to the selecting circuit IFS, a vacuum tube detector D whose grid circuit is biased by means of a C battery or otherwise in such manner that, in the absence of a high frequency carrier current, no current flows in the plate circuit. The plate circuit is connected to the B battery through the line winding of the receiving relay RR. The receiving relay also is provided with a biasing winding which tends to normally hold the armature of the relay against its spacing contact. The apparatus is so adjusted that when a carrier current corresponding to a marking signal is applied to the grid of the detector D, the plate current flowing through the line winding of the receiving relay will produce a pull on the armature, tending to shift it to its marking contact with a force about twice as great as that produced by the biasing winding.

The bnck7 channel includes a selecting circuit IFS which may be similar to the selecting circuit IFS previously described. This selecting circuit should be selective of a frequency closely related to the carrier frequencies assigned to the several signal channels as they appear after being stepped down by the demodulator DD. The buck channel also may include an intermediate frequency amplifier IFA of any known type such as, for example, a vacuum tube amplifier, for the purpose of increasing the magnitude of the bucking impulses, as will be described later.

The selecting circuit IFS and amplifier IFA of the buck channel are common to the neutralizing impulses of several receiving channels, where the system is multiplexed. The output circuit of the amplifier IFA is connected to a plurality of detectors such as D', one corresponding to each receivingl channel. The detector D may be in all respects similar to detector D, and has its grid biased in the manner already described so that no current normally Hows in the plate circuit. The plate circuit of the detector D is connected to the B battery over the contacts of a reversing relay PR and through the buck winding of the receiving relay RR.

The reversing relay PR, which is a polar relay, is connected in a leak circuit associated with the receiving leg, said leak circuit comprising a resistance R1 in series with a parallel combination of resistance R2 and capacity C. By this arrangement the reversing relay is controlled by the armature of the receiving relay as it is shifted from marking to spacing, and vice versa. By means of its armature the reversing relay PR reverses the direction of the plate current of the detector D through the buck winding so that thezimpulse of current in the plate circuit caused by an atmospheric disturbance will always be in such a direction as to tend to hold the armature against the contact upon which it rests at the'moment the disturbance occurs. The resistances R1 and R2, together with the capacity C, serve to accelerate the action of the polar relay PR. At the berinning of a signal wave originated by the armature of the receiving relay RR in the receiving leg, the capacity C acts substantially as a short-circuit for the resistance R2, so that the current through the reversing relay will have a sharp peak at the beginning of the pulse. As the capacity C charges up, current begins to flow through the resistance R2, so that the current through the reversing relay drops off to a value deterlnined by the resistances Rl and R2 in series with the resistance of the polar relay PR.

The operation is as follows: Assuming that the system is operated on a multiplex basis, the selecting circuit RFS is arranged to freely pass a band of frequencies wide enough to encompass the frequencies assigned to the several receiving channels and, in addition, the frequency assigned to the buck channel. The selecting device discriminates, so far as possible, against frequencies lying outside the desired band. At radio frequencies, however, it is impracticable to design a selecting circuit, such as RFS, which will sharply discriminate between the desired band and those frequencies lying outside, so that the desired band, together with some unwanted frequencies, will necessarily be impressed upon the amplifier RFA. The amplified waves are then applied to the demodulator DD, together with the frequency generated by the heterodyne source G. This frequency is so related to the received band as to produce in the output of the demodulator a side band comprising sub-hands corresponding to each of the signal bands but shifted in the frequency spectrum to a low position. The waves from the output of the demodulator DD are then amplified by the intermediate frequency amplifier IFA.

The waves appearing in the output of the intermediate frequency amplifier comprise a frequency range corresponding in width to that passed by the selecting circuit RFS, but as this band is now located at a lower point in the frequency spectrum, it is relatively much easier to sharply select the desired frequencies from the unwanted frequencies. The selecting circuit IFS, therefore, selects from the intermediate frequency range the desired sub-band of frequencies assigned to its particular channel and, similarly, the selecting devices for the other signal receiving channels select their particular subbands. Y

If no atmospheric disturbance is occurring, no frequency Will appear corresponding to the selecting circuit IFS of the buck channel, and hence the buck channel takes no part in 'the operation. The band selected by the selecting circuit IFS, however, is impressed upon the detector D so that in the plate circuit of the latter a current Will flovv through the line Winding of the receiving relay RR for each group of high frequency waves corresponding to a marking signal, but no current will flow during the spacing signal. The armature of the receiving relay RR is accordingly shifted from marking to spacing, and vice versa, as the received signals change in character, thereby transmitting corresponding direct current signals to the receiving leg.

If an atmospheric disturbance occurs, such disturbance Will usually manifest itself as a sharp sudden Wave Whose frequency components extend over a very Wide range. If none of these frequency components falls Within the range of frequencies corresponding to the several signal channels being received, the intermediate frequency demodulating portion of the system Will result in a practically complete suppression of the disturbance because the disturbing Waves Will be shifted down in the frequency spectrum by the demodulator DD so that the sharply selective circuit, such as IFS, will be able to reject the undesired frequency components. If, however, there are frequency components in the disturbing Wave which fall Within the range of frequencies covered by signal channels, such Waves will be passed by the selecting circuit RFS which may also partially, though perhaps not entirely, suppress other undesired Waves lying on either side of the range assigned to the useful signals. These components are amplified and demodulated by the amplifier RFA and the demodulator DD, along with the desired signal waves, and with the latter are stepped down in the frequency spectrum. The amplifier IFA then amplifies the stepped-down Waves and each selecting circuit, such as IFS, selects its band withstanding of frequencies, together with such interfering com onents as fall Within its particular range. Similarly, the selecting circuits IF S of the interfering Wave corresponding to its assigned range but Will select no carrier or signal components as no such components corresponding to the buck channel are transmitted fromk the distant station.

The effect of the interfering frequencies selected by the circuit IFS Will depend upon Whether a marking signal or spacing signal is being received a spacing signal is being received so that normally no plate current is flowing through the line Winding of the relay RR, the effect of the disturbing frequencies is to cause a plate current to.floiv, thereby causing a false marking signal during the spacing interval. If, however, a marking signal is being received, the effect of the interfering frequencies will cause either an increase or a decrease in the plate current, depending upon the magnitude of the interfering Wave. 'An increase in the plate current of the detect'orD is without harmful effect, so far as causing a false signal is concerned, but if the plate current is materially decreased ,as a result of the interfering Wafe a false spacing signal might occur during the marking interval.

'lhe effect of the bucking channel, however, is to prevent false signals of either of the types above referred to. Regardless of Whether a spacing signal or a marking signal is being received by the signal channel or channels, the buck7 channel receives the interfering Wave components corresponding to the selecting circuit IFS, and these Waves are amplified by the amplifier IFA and applied to the detector D of thc particular channel illustrated (and to other similar detectors when the system is multiplexed). This causes a current to How in the plate circuit of the detector D Where normally no current should flow. This pulse of current passes through the buck7 Winding of the receiving relay ARR over the contacts of the reversing relay PR- in a direction depending upon Whether a marking signal is being received or a spacing signal. If a marking signal 1s being received the reversing relay Will have its armatures resting against its front contacts, as shown, so that the pulse of current from the plate circuit of the detector D will be in such a direction as to tend to hold the armature of the receiving relay RR upon its marking contact notthe decrease in the plate currentthrough the line winding. If a spacing signal is being received the reversing ,univ Will have its armatures resting against 11,-; back contacts. and the direction of current through the buck Winding will be reversed so as t0 tend to hold the armature against the spacing contact notwithstanding the false buck channel Will select components' at the time of disturbance. If Y signal current through the line winding. The amplifier IFA is provided in order to increase the amplitude of the bucking impulse which results from an interfering wave, so that even when the components of the interfering wave which correspond to the bucking channel are relatively weak as compared to those components which enter the signal channels, the current impulse in the plate circuit of the detector D will be large enough to hold the armature upon the Contact against which it rests in spite of the false signal current through the line winding of the relay RR.'

.It will be obvious that the general princlples herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. In a high frequency telegraph system, the method of suppressing atmospheric interference which consists in selecting a band of frequencies corresponding to the desired slgnal while partially suppressing interfering frequency components lying without said band, stepping down the resultant frequency components to a lower range in the frequency spectrum, selecting the band corresponding to the ydesired signal in its lower range while practically suppressing frequencies outside the band, detecting from the selected band slgnal pulses to actuate a receiving relay, selecting from frequenciesin the lower range a component corresponding to the interfering wave and having a frequency in the neighborhood of the desired signal band in the lower range, translating the selected component into a pulse of current, and producing by said pulse of current a force tending to hold the armature of the receiving relay upon the contact against which it happens to rest when the interfering wave occurs.

2. In a high frequency telegraph system, means to select at high frequency a band of frequencies corresponding to a desired signal while at least partially suppressing interfering frequencies outside the band, means to step down the resultant frequency components to an intermediate frequency range, means to select in the intermediate frequency stage the desired signal band to the exclusion of interfering frequencies without the band, means to detect from the selected signal band direct current pulses corresponding to telegraph signals, a receiving relay responsive to said pulses, a buck channel including means to select at the intermediate frequency stage interfering components of frequency closely related to the desired signal band, means to produce therefrom a pulse of current, and means to produce from said pulse a force tending to hold the armature of the receiving relay upon the contact against which it happens to rest when the interfering wave occurs.

3. In a high frequenc telegraph system, means to select at high requency a band of frequencies corresponding to a desired signal while at least partially suppressing interfering frequencies outside the band, means to step down the resultant frequency components to an intermediate frequency range, means to select in the intermediate frequency stage the desired signal band to the exclusion of interfering frequencies without the band, means to detect fromthe selected signal band direct current pulses corresponding to telegraph signals, a receiving relay responsive' to said pulses, a buck channel including means to select at the intermediate frequency stage interfering components of frequenfy closely related to the desired signal ban means to produce therefrom a pulse of current, an auxiliary winding of said receiving relay through which said pulse is caused to flow, and means to reverse the effective direction of said pulse with reversals in the character of the signal so as to hold the armature of the relay -upon the contact against which it happens to rest when the interfering wave occurs.

4. In a high frequenc telegraph system, means to select at high requency a band of frequencies corresponding to a desired signal while at least partially suppressing interfering frequencies outside the band, means to step down the resultant frequency components to an intermediate frequency range, means to select in the intermediate frequency stage the desired signal band to the exclusion of interfering frequencies without the band, means to detect from the selected signal band direct current pulses corresponding to telegraph signals, a receiving relay responsive to said pulses, a buck channel including means to select at the intermediate frequency stage interfering components of frequency closely related to the desired signal band, means to produce therefrom a pulse of current, an auxiliary winding of said receiving relay through which said pulse is caused to flow, a reversing relay controlled by said receiving relay, and means controlled by said reversing relay to effectively reverse the direction of flow of said 4pulse through said winding as the character of the received signal changes.

5. In a high frequency telegraphsystem, means to select at high frequency a band of frequencies corresponding to a desired signal while at least partially suppressing interfering frequencies outside the band, means to step down the resultant frequency components to an intermediate frequency range, means to select in the intermediate frequency stage the desired signal band to the exclusion of interfering frequencies without the band, means to detect from the selected signal band direct current pulses corresponding to telegraph signals, a receiving relay responsive to said pulses7 a buck channel including means to select at the intermediate frequency stage interfering components of frequency closely related to the desired signal band, means to amplify said interfering components, means to produce therefrom an amplilied pulse of current, and means to produce from said pulse a force tending to hold the armature of the receiving relay upon the contact against Which it happens to rest when the interfering Wave occurs.

In testimony whereof, I have signed my name to this specification this 24th day of November, 1928.

LEO A. KELLEY. 

